Much attention has been given in recent years to the development of ultrasonic systems for producing real time images of internal portions of the human body. In one system an array of transducers for converting short pulses of alternating current electrical waves into corresponding pressure waves is placed in contact with the body. By choosing the relative times of application of the pulses of electrical waves to the transducers, the pressure wave for each pulse can be formed into a beam extending in any desired direction. The direction of the beam of pressure waves for each pulse can be changed so as to effectively scan a sector. As the pulses of pressure waves pass through the body tissue, a portion of their energy is reflected whenever they encounter tissue having a different acoustic characteristic. An array of receiving transducers that convert the reflected pressure waves into corresponding electrical waves is dynamically focused (from a minimum to a maximum range) along the direction of each transmitted beam.
Focusing of the array of receiving elements at a given point requires that the few cycles of the alternating current wave derived by each of the transducers from the pulses of pressure waves reflected from that point be superimposed in phase at a summing point. This produces a strong signal, whereas reflections of pressure waves from other points produce electrical waves that reach the summing point with random phase relationships. Because the distances between any desired focal point and the various receiving transducers are usually different, the reflections arrive at the transducers at different times. It is therefore necessary to introduce compensating delays between each transducer and the summing point so that the total time between reflection of a pressure wave at the focal point and the arrival of the corresponding electrical wave at the summing point is the same regardless of which transducer is involved. The compensating delays are varied dynamically so that the focal point moves from minimum to maximum range along each direction of the transmitted pulses as previously described.
In some previous instruments, for example, separate tapped delay lines have been connected to each transducer and the delay required for focusing is determined by connecting an appropriate tap to the summing point. Each delay line must be capable of providing the maximum amount of delay required by the transducer to which it is connected, so that the sum of the delays for all the delay lines is large. Inasmuch as each line has a high individual cost, an instrument constructed in this manner is necessarily expensive.
This invention basically involves using one long delay line, the master line, that is effectively tapped so that a signal injected into each tap experiences a different delay. A plurality of incremental delay means having small variable delays are provided. Each is connected between one or more of the transducers and a selected tap on the master line. The small variable delay can be provided by any means known to those skilled in the art, e.g., a line of lumped inductances and/or capacitors with either or both variable, means for combining delay lines of small but different delays by switched interconnections, and by a simple tapped short delay line. In the description that follows the small or variable delays are provided by a tapped delay line, but other equivalent controllable incremental delay means may be used.
The taps on the master delay line to which the transducers are connected through the small variable delays are selected so as to focus the array along a desired scan angle or direction and the small variable delays are changed during a scanning of that direction so as to shift the focus of the array from minimum to maximum range.